1. Field of the Invention
This invention relates to an optical recording medium and a manufacturing method thereof. More particularly, it relates to an optical recording medium a recording layer of which is heated by illumination of light, such as laser light, to raise its temperature to produce phase change therein to record or erase the information to optically read out information signals, and a manufacturing method thereof.
2. Description of the Related Art
In the field of data recording, investigations into the optical data recording system are proceeding briskly. This optical data recording system has a number of advantages, such as non-contact recording/reproduction, a recording density higher by one digit of magnitude than in the magnetic recording system, and adaptability to memory configurations such as read-only, write-once or overwrite memory configurations, and is finding a wide field of application ranging from the industrial usage to the domestic usage as a system enabling realization of an inexpensive large-capacity file.
The optical data recording system, adapted for the overwrite memory configuration, may be enumerated by a magneto-optical disc and a phase-change optical disc.
The phase change optical disc, associated with the overwrite memory configuration, has the following structure: That is, a transparent dielectric film of ZnS etc is formed on a major surface of a transparent substrate, and subsequently a phase-change recording film of ZnS etc and a reflective film, such as an aluminum film, are formed thereon.
The chalcogen-based alloy material may be enumerated by a Ge--The--, Ge--The--Sb--, In--Sb--The-- or Ge--S--Te-based materials. These materials permit information recording/erasure by exploiting transition between the crystal-amorphous and crystal-crystal phases. Of these materials, those having a specified composition of the Ge--Sb--Te-based material, as disclosed in JP Patent Kokai JP-A-62-53886, 3-80635 and 63-225934 and in JP Patent Kokoku JP-B-8-32482, or those having a specified composition of the Ag--In--Sb--Te-based material, as disclosed in JP Patent Kokai JP-A-6-166268 and 4-232779, are well-known.
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When recording the information on the phase-change optical disc, the light, such as the laser light, is illuminated from the transparent substrate side on the phase-change recording film to raise its temperature in a spot fashion to change the state of a portion of the phase-change recording film to effect the recording. The above-mentioned material is amorphous on rapid heating or quenching, and is crystallized on gradual cooling, depending on the heating temperature. Thus, the information is recorded by forming an amorphous portion and a crystalline portion.
For reproducing the information from the phase-change optical disc, the laser light, for example, which will not produce changes in the phase-change recording film, is illuminated to the phase-change recording film, from the side of the transparent substrate, to detect the information depending on the difference between the return light from the crystalline portion and that from the amorphous portion.
Meanwhile, the above-mentioned phase-change optical disc cannot be said to be sufficient in signal properties or recording durability for high linear velocity and for high recording density, such that the properties of the phase-change optical disc need to be improved further.
Also, the phase-change optical disc is adapted to the overwrite memory configuration, so that the phase-change optical disc is to be improved further in recording/erasure characteristics and in repetition durability.